The present invention relates to a polishing tool for processing an optical surface of a spectacle lens, having a carrier body and a polishing film, an elastic layer being arranged between the polishing film and the carrier body.
Furthermore, the present invention relates to an apparatus for processing optical surfaces having a polishing tool of this type.
Spectacle lenses are conventionally produced from a semi-finished product by material-removing or abrasive processing of what is known as the prescription surface or surfaces. The optically relevant shape of the spectacle lens is therefore fixed. Finally, the spectacle lens is also polished; no change in the optical properties of the spectacle lens may be brought about by the polishing, however.
In order to polish a surface of a spectacle lens, a polishing head is usually used which has a polishing tool, the polishing surface of which is adapted at least approximately to a shape of that surface of the spectacle lens which is to be polished. The polishing tool and/or the spectacle lens are/is mounted in an articulated manner, in particular by way of a ball joint, and are guided relative to one another with a predefined movement sequence, usually with the aid of CNC systems.
When spherical or toric spectacle lenses are polished, it is less problematical, on account of the relatively simple shape of the surface to be polished, to find a suitable polishing tool of complementary configuration which can be guided over the surface with simple movement sequences which does not bring about any impermissible deformations. On account of the multiplicity of possible spherical or toric spectacle lenses, it is merely required to keep a corresponding multiplicity of polishing tools available.
Similar polishing tools of this type are shown, for example, in documents DE 101 00 860 A1, EP 0 567 894 B1, DE 44 42 181 A1, DE 102 42 422 or DE 101 06 007 A1.
A common feature of these polishing tools is that a pressing rigidity which extends in a radial direction of the polishing tool is either constant or decreases slightly from the inside to the outside. The flexural rigidity of the polishing tool therefore decreases from the inside to the outside in a direction, in which a force is loaded onto the spectacle lens by the polishing tool, or is constant.
This is sufficient for spherical and toric, that is to say simply shaped, surfaces. When what are known as freeform surfaces or aspherical or point symmetrical, arbitrarily shaped surfaces are polished, polishing tools of this type cannot be used without problems, in contrast.
Aspherical or point symmetrical surfaces and freeform surfaces have curvatures which change over the surface. In particular, freeform surfaces of this type are used in individual spectacle lenses which are adapted to a user. During the polishing processing of freeform surfaces of this type, the polishing tool moves at least over a part of this irregularly curved surface. The flexural stability or elasticity of the polishing tool therefore has to be capable of being adapted to the respective local curvature, to be precise in such a way that the polishing pressure is as constant as possible over the contact surface. Only then does this result in a definable constant abrasion, and the polished surface is polished uniformly. If this is not ensured, the surface or the topography of the freeform surface is deformed and its optical quality is impaired as a result.
For mass polishing processing of freeform surfaces and also of spherical or toric spectacle lenses from plastic materials, inexpensive polishing tools of simple construction are used according to the currently known prior art. The plastic materials of the spectacle lenses are, for example, a polycarbonate, for instance CR 39 which is marketed by the company PPG Industries, Pittsburgh, USA. The polishing tools usually comprise an at least three layer construction. The polishing tools have at least one fixed basic body which faces the tool spindle which rotates the polishing tool, and on which basic body a foam layer or other elastic layer is adhesively bonded or attached. A polishing film which faces the spectacle lens or workpiece is in turn provided on said foam layer. On account of the elastic deformability of the foam layer, the polishing film can be adapted by a certain amount to the topography of the spectacle lens surface to be polished. In order to assist the capability of the polishing surface of the polishing tool to be adapted to the surface of the spectacle lens, the polishing tools are generally smaller than the spectacle lens. The polishing abrasion is produced with the aid of an abrasive polishing liquid by the relative movement of the polishing tool which is loaded with pressure.
An example of a polishing tool similar to this type of polishing tools which also makes the application of a polishing liquid possible is shown in document DE 10 2005 010 583 A1.
For a polishing tool which is high quality under optical aspects, it is important that the polishing force which is applied to the glass by the tool decreases in the edge region of the polishing surface of the polishing tool toward the outside, ideally moves continuously toward zero. If this is not ensured sufficiently, visible spiral structures which are caused by the edge of the polishing tool, impair the quality of the spectacle lens surface and can even make it unusable are produced on the polished glass.
As a solution for this, it has been proposed in the prior art, for example, to configure foam layers with a lower hardness in the edge region, for instance by an increase in the material thickness, and as an alternative or cumulatively to allow the polishing film to protrude beyond the edge of the foam layer. A similar solution is shown, for example, in document EP 1 644 160 B1.
The aim of a pronounced decreasing polishing force in the edge region of the tool cannot be achieved sufficiently, however, by way of this proposed solution. As a result, depending on the selection of the polishing parameters, cosmetic defects are nevertheless produced on the spectacle lens. These faults on the optical surface can be tolerated or not as a function of the quality demands made on the spectacle lenses to be produced. These problems are reinforced by material fatigue in the case of long-lasting use of the polishing tool. If the material is subject to fatigue in the zone of the greatest loading, which zone lies between the centre and the edge in the above-described solutions, the polishing force is in turn increased in the edge zone and causes the undesirable effect in a reinforced manner.
It is a further observed effect that the polishing foil becomes wavy after multiple use of the polishing tool and accumulates in the direction of the glass surface, as a result of which polishing errors can be produced. This can take place by the diffusing of liquid polishing medium into the edge region of the polishing film and the swelling of the porous material which is caused as a result. Cosmetic defects can also be caused by polishing medium which is caked and embedded in the edge region.
It has been proposed as a further solution approach to use a polishing film with a relatively low material thickness and a resulting relatively low mechanical stability.
Finally, this solution permits only material thicknesses which oppose a requirement for high loadability and a long service life of the polishing film. Sufficiently stable polishing films are required for high efficiency of the polishing process and high resistance to mechanical wear.
Finally, polishing tools with different, pneumatically actuable pressure zones have also been proposed. Ultimately, however, said polishing tools require a structurally complicated construction which is once again expensive and prone to maintenance. Furthermore, the pressure zones cannot have as fine a resolution as desired, with the result that there is frequently, despite everything, no sufficient control over the pressure conditions, in particular in critical edge regions. One example for a similar approach of this type is shown in document US 2006/0094341 A1.
It is therefore an object of the present invention to provide a polishing tool for the improved processing of optical surfaces, in particular freeform surfaces.